Characteristics of quasi‐monochromatic ULF waves in the Venusian foreshock

The statistical properties of ULF waves observed upstream of Venus foreshockare investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi-monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2-3 times of the local proton cyclotron period. The transverse power dominates the power spectrum and most of the waves display nearly circular or slightly elliptical polarizationin the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B0 for parallel component (δB||/B0 less than 0.3), while the range of relative amplitudes for perpendicular component δB⊥/B0 is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshockform an important energy source for the generation of the waves.